Enteropathogenic E. coli (EPEC), an intestinal pathogen that causes diarrhea, is responsible for significant morbidity and mortality, especially among children below 5 years of age in developing countries. The precise mechanism(s) by which EPEC causes diarrhea is currently not known. The bacteria elaborate a syringe-like type III secretion system that delivers key virulence factors directly into host intestinal epithelial cells. During the initial phase of infection, EPEC appears to specifically limit the death of the underlying epithelial cells. The focus of this proposal is the suppression of host cell death by the type III-secreted protein EspZ, which is unique to EPEC and related pathogens. Strains lacking EspZ are highly impaired for colonization and disease in animal models of infection. We hypothesize that EspZ interacts with host proteins to suppress premature death of infected enterocytes and, thereby, plays a key role in intestinal colonization by A/E pathogens. We propose to (1) validate the interaction of EspZ with host cell partners, define EspZ interaction domains, and generate specific EspZ mutants deficient for interaction(s), (2) characterize the role of EspZ and its interaction partners in activating signaling pathways that contribute to the survival of host cells, and (3) define the role of EspZ and its interaction partners in pathogenesis using animal models of infection. Understanding the key role of EspZ, and of host survival to pathogenesis, could lead to future efforts to inhibit EspZ function, and thereby control EPEC-associated diarrhea.